1. Field of the Invention
The present invention relates generally to the field of direct oxidation fuel cells, and in particular to a system and method for removal/oxidative decomposition of un-reacted fuel from a fuel storage container for use in a direct oxidation fuel cell and direct oxidation fuel cell system.
2. The Prior Art
Fuel cell technologies present opportunities for the commercial development of long-lasting power sources for portable power and electronics applications. With the trend toward greater portability of a wide array of consumer electronics, some fuel cell technologies offer promising alternative power sources to meet the increased demand for portable power. Fuel cells can potentially replace or favorably compete with the various types of high power density batteries presently used in consumer electronics, such as nickel metal-hydride and lithium ion battery systems, as well as relatively inexpensive alkaline batteries. These types of batteries are less than satisfactory power sources for such consumer electronics as laptop computers and cellular phones either due to their low power density, short cycle life, rechargability, or cost. In addition, all these types of batteries present environmental safety concerns and costs for proper disposal.
Fuel cell systems are electricity-generating devices that convert chemical energy into electricity via a simple electrochemical reaction involving a fuel reactant such as natural gas, methanol, ethanol, or hydrogen, and an oxidizing agent typically ambient air or oxygen into useable electrical energy. Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion before it is introduced into the cell) and “direct oxidation” systems in which the fuel is fed directly into the cell without internal processing. Most currently available stationary fuel cells are reformer-based fuel cells. However, fuel processing requirements for such cells limits the applicability of those cells to relatively large systems.
Direct oxidation fuel cells, wherein the fuel reactant is fed directly into the fuel cell without internal modification or oxidation, are typically constructed of an anode diffusion layer, a cathode diffusion layer, and an electrolyte, such as a protonically conductive, electronically non-conductive membrane (“PCM”), that is disposed between the anode and cathode diffusion layers. Fuel reactant is introduced into the fuel cell anode and is presented to a catalytic layer intimately in contact with the anode face of the PCM. The anode catalyst layer separates hydrogen from the fuel reactant into protons and electrons as a result of oxidation, releasing hydrogen ions (protons and electrons) from the fuel reactant molecule. Upon the completion of a circuit, protons generated by the anodic catalytic reaction pass through the membrane electrolyte to the cathode of the fuel cell. Electrons generated by anodic oxidation of fuel molecules cannot pass through the membrane electrolyte, and seek another path through the load which is being powered. The electrons flow away from the anode catalyst, through the anode diffusion layer, through a load (typically via a current collector), through the cathode diffusion layer and to the cathode catalyst layer where the electrons combine with protons and oxygen to form water.
As long as adequate supplies of fuel reactant and an oxidizing agent are available to the fuel cell, the cell can generate electrical energy continuously and maintain a desired power output. Hence, fuel cells can potentially run laptop computers and cellular phones for several days rather than several hours, while reducing or eliminating the hazards and disposal costs associated with high density and alkaline batteries. A further benefit is that a fuel cell runs cleanly producing water and carbon dioxide as by-products of the oxidation/reduction of the fuel reactant. The challenge is to develop fuel cell technology and to engineer direct fuel cell systems to meet the form and operation requirements of small-scale or “micro” fuel cells for portable electronics applications.
Direct methanol fuel cell (“DMFC”) systems are often multi-cell “stacks” including a number of single fuel cells joined to form a cell stack to increase the voltage potential to meet specific electrical power requirements. The feasibility of DMFC systems as alternative power sources for portable electronics applications will depend upon the reduction of the size of the overall system to meet demanding form factors, while satisfying the necessary power requirements for electrical power applications.
At present, prior art DMFC systems typically operate in several configurations, as disclosed, for example in U.S. Pat. Nos. 5,992,008, 5,945,231, 5,795,496, 5,773,162, 5,599,638, 5,573,866 and 4,420,544. As fuel cell technology is developed, other variations and configurations may develop with their own advantages and disadvantages.
In a DMFC, it is necessary to provide sufficient quantities of fuel (typically neat methanol or a mixture of water and methanol) to the catalyzed anode face of the PCM, and oxygen to the catalyzed cathode face of the PCM. Failure to allow sufficient quantities of the reactants to be introduced to the PCM results in the cessation of the reactions that generate electricity in a fuel cell.
Methanol is an attractive fuel for fuel cell systems, and as with most other appropriate fuels, it is flammable under certain conditions, and it may have detrimental effects on health if ingested.
In a typical fuel cell system, residual fuel remains in the spent anode effluent storage or in a container which is to be removed when practically, but not completely emptied. Keeping the fuel inside such a container after being discarded may raise some health or safety concerns. There remains a need, therefore, for a system and method for safe removal of such residual fuel from the spent fuel storage container or containers of a fuel cell and fuel cell system.